1. Field of Industrial Applicability
The present invention generally relates to a projection exposure apparatus for photolithography, and, more particularly, to a projection exposure apparatus adapted to reduce a decrease in performance due to radiation-induced damage of lens material(s) comprised therein.
2. Prior Art
Lithographic processes are commonly used in the manufacture of semiconductor elements, such as integrated circuits (ICs), LSIs, liquid crystal elements, micropatterened members such as thin-film magnetic heads and micromechanical components. A projection exposure apparatus used for lithography generally comprises an illumination optical system with a light source and a projection optical system. Light from the illumination optical system illuminates a patterning structure (mask) with a given pattern and the projection optical system transfers an image of the patterning structure pattern onto a photo-sensitive substrate. The image of the mask may be reduced in size by the projection optical system so as to project a smaller image of the patterning structure onto the substrate.
The trend towards ever more sophisticated semiconductor devices requires semiconductor elements of smaller size and higher complexity which, in turn, makes higher demands on the resolution achievable in projection optical systems. Improved resolution is generally achieved by increasing the numerical aperture of the projection optical system as well as decreasing the wavelength of the exposure radiation (illumination light), with recently used illumination light having wavelengths of 248 nm and below.
However, the transition to ever shorter wavelengths is associated with a number of problems. In particular, the number of suitable materials for lenses in the projection optical system as well as in the illumination optical system having a large enough transmittance at short wavelengths is very limited. Furthermore, the materials that are satisfactory in terms of transmittance often suffer damage upon exposure to radiation. Whilst it was, at first, observed that certain lens materials will undergo compaction, i.e. densification, it was later noticed that some materials show the opposite effect, i.e. rarefaction, which involves an expansion of the lens material.
Fused silica (sometimes referred to as “quartz” in a more general manner) is the most common material used in recently introduced projection exposure apparatus employing short wavelengths for exposure (193 nm and 248 nm being the most common). Upon exposure to high intensity radiation, exposed areas of a lens of a given material have been found to undergo a change in density, in particular densification or rarefaction. The change in density of an exposed area in a lens in a projection exposure apparatus can generally be assumed to have a detrimental effect on the optical properties of the lens. In particular, wavefront distortion is indicative of densification or rarefaction and can be measured and determined by suitable interferometric methods, for example. An increase in density, for instance, of the lens material shortens the physical path through the material, but also alters the refractive index, which is generally increased to a greater extent, so that the net effect is an increase in the optical path. For rarefaction phenomena, the opposite applies. Usually, densification and rarefaction are quantified in terms of the change of the product of refractive index and path length, commonly referred to as optical path length difference OPD.
The susceptibility of fused silica materials to UV-induced damage is correlated with the materials' chemical and physical properties, which, in turn, are closely linked to methods of manufacturing and/or treating the material(s).
A lens material that is transparent to UV-radiation and which has, at least so far, not been found to be subject to such structural alterations that are associated with changes in optical properties is calcium fluoride, CaF2. However, for calcium fluoride to be suitable for use in optical lenses, it needs to be in the form of single crystals which are not only costly but also technically difficult to manufacture so that the resulting limited supply somewhat constrains its practical use.
Accordingly, fused silica material is still the most widely used option in terms of lens materials suited for photolithography with UV-radiation.
The observation of the above described phenomena has been an incentive for both materials scientists and projection exposure apparatus designers to find solutions to the problems created by the structural change of the respective lens material manifesting itself as a density and refractive index change over the lifespan of a projection exposure system.
In U.S. Pat. No. 6,295,841 B1 by Allan et al., a method of precompacting fused silica and a method for making a fused silica stepper lens are disclosed, the entire content of which is incorporated herein by reference.
In U.S. Pat. No. 6,339,505 B1 by Bates, a method for radiation projection and a lens assembly for semiconductor exposure tools are disclosed, the entire content of which is incorporated herein by reference. In particular, a lens assembly comprising a first lens element of a material that densifies upon exposure to radiation and a second lens material that rarefies upon exposure to radiation are described. The lens materials are supposed to be selected such that the change in optical path length difference of one lens will exactly compensate for that of the other lens. Exact compensation, however, has proven difficult, if not almost impossible to achieve in practice.
There remains a need for a projection exposure system which is adapted to reduce a decrease in performance of the optical system comprised therein due to radiation-induced damage to the lens material(s), and in particular a radiation-induced change in density of the lens material(s).